A transformerless inverter for feeding electric energy from a DC current source into an AC power grid comprising an inverter bridge and a DC/DC converter connected upstream of the inverter bridge is known from EP 2 023 475 A1. Here, the DC/DC converter provides at least two bipolar output voltages, which are additively superimposed between the input lines of the inverter bridge with regard to a grounded center point. Particularly, each resonant circuit is branched and comprises two partial resonance capacitances that are each connected to two oppositely oriented rectifier diodes at their output and that are thus alternately connected in an electrically conductive way to the lines of one part of a divided intermediate link to which the inverter bridge is connected. In this way, the known inverter comprises a basic conversion of the DC voltage present between the input lines of the inverter into an intermediate link voltage present at the input lines of the inverter bridge of 1:n, n being the total number of partial resonance capacitances towards which the resonant circuits are branched. In other words, n is the number of the parts of the divided intermediate link out of which the inverter bridge is fed. This basic conversion of the input DC voltage proves unfavourable when the known inverter is used for feeding electric energy from a photovoltaic panel into an AC power grid, if the DC voltage provided by the photovoltaic panel is already higher or at least nearly as high as the peak voltage of the AC power grid. The photovoltaic panels that are generally used today sometimes provide such high DC voltages that the peak voltage of an AC power grid to be fed is often exceeded by a multitude.
Not all embodiments of the transformerless inverters known from EP 2 023 475 A1 comprise a galvanic separation between the input lines of the inverter and the input lines of the inverter bridge. Instead, in some of the embodiments, grounding of the center point of the divided intermediate link and of one of the input lines of the inverter is provided, which grounding thus extends beyond the intermediate link. In the embodiments of the known inverter that provide a galvanic separation between the input lines of the inverter and the input lines of the inverter bridge in a capacitive way, a center point of the divided intermediate link which is connected to the input lines of the inverter bridge via capacitances is connected to a center point between the input lines of the inverter that is also connected to these input lines via capacitances. By means of this connection, a passive AC current backflow path is provided which, because of its pure capacitive connection to the input lines of the inverter, does not delete the galvanic separation between the input lines of the inverter and the input lines of the inverter bridge.
In the embodiments of the transformerless inverter known from EP 2 023 475 A1, in which the input lines of the inverter are galvanically separated from the input lines of the inverter bridge, a reference potential for the input DC voltage may be freely selected. However, there is the danger that the summation current of the currents flowing over the resonant circuit and the passive AC current backflow path is not zero so that undesired compensation currents flow via ground, which may be high.
A switched-mode power supply disclosed in WO 01/80411 A1 comprises an input circuit for periodically switching an input voltage or an input current on and off with a switching frequency, and a transmission circuit connected thereto. The transmission circuit is formed by a bandpass circuit consisting of at least one capacitor and at least one inductor, the resonant frequency of said bandpass circuit lying outside, especially above, the switching frequency of the input circuit.
There is a need for a transformerless inverter which is better suited for DC current sources that provide a high DC voltage as compared to the peak voltage of the AC power grid to be fed, and that provides further adaptation options.